A Paper on Ground Improvement Techniques

A Paper on Ground Improvement Techniques May 2nd, 2011 by Anvesh Kumar | Posted under Geotechnical Engineering, Engineering, ppt ppt..

Ground improvement is the most imaginative field of geotechnical engineering. It is a field in which the engineer forces the ground to adopt the project‟s requirements, by altering the natural state of the soil, instead of having to alter the design in response to the ground‟s na tural limitations. The results usually include saving in construction cost and reduction of  implementation

time.

There are number of techniques available for improving the mechanical and engineering properties of the soil. However, each technique has some limitations and suit abilities to get maximum improvement in the soil conditions with minimum effort. Some of the important techniques

are

discussed

in

this

paper.

To improve the strength of the soils, especially in case of granular type of soils, COMPACTION METHODES are found as best methods among all type of techniques. Based on the mechanism applied for compacting the soil, it is sub divided into different methods like dynamic compaction, blasting, vibro techniques …etc.These are briefly discussed in tthis his paper.

When there are some limitations encountered for applying the above technique, grouting techniques, stabilization of soil using different admixtures can be adopted effectively which can bring variations in the soil conditions. The various types of above techniques are briefly discussed in this paper.

Finally, recent advancements in ground improving techniques using GEOTEXTILES, ELECTRIC TREATMENT METHODES are also briefly discussed in this paper. These techniques

are

widely

used

in

these

days.

INTRODUCTION

Large civil engineering projects are being executed in all over the country in order to enhance the infrastructure of the country. Infrastructure facilities have to be often built at sites where the soil conditions are not ideal. The insitu soil characteristics of a construction site are different from those desired, and almost always far from ideal for a designed need. With i ncreased urban development, site with favorable foundation conditions became depleted. At times the civil engineer has been forced to construct structures at site selected for reasons other than soil conditions. Thus it is increasingly important for the engineer to know the degree to which soil properties may be improved or other alterations that can be thought of for construction of an intended structure at stipulated site.

If unsuitable soil conditions are encountered at the site of a proposed structure, one of the following four procedures may be adopted to insure satisfactory performance of the structure.



By pass the unsuitable soil by means of deep foundations extending to a suitable bearing material.



Redesign the structure and it‟s foundation for support b y the poor soil. This procedure may not be feasible or economical.



Remove the poor material and either treat it to improve and replace it (or)

substitute for it with a suitable material.



Treat the soil in place to improve its properties. Rigid foundations such as piling present a solution but these are often expensive. In such circumstances, ground improvement using different techniques offers a proved and economic solution. At present a variety of soil improvement techniques are available for making soil to bear any type of  structure on it and also for mitigation of seismic hazards. The costs of these methods vary widely and the conditions under which they can be used are influenced by nature and proximity of structures and

construction

facilities.

GROUND IMPROVEMENT TECHNIQUES

On the basis of mechanism by which they improve the engineering properties of soil, the most of  common of these can be divided into the following major categories. These are



Densification techniques.



Reinforcement techniques.



Stabilization techniques.



Miscellaneous methods

Apart from the methods listed above, there are some other simple methods like removal and replacement of soil. In this paper these are discussed first before taking up above techniques.

1. REMOVAL AND REPLACEMENT OF SOIL

One of the oldest and simplest soil improvement methods is to simply excavate the unsuitable soil and replace them with compacted fill. This method is often used when the problem the soil is that it is too loose. In that case, the same soils used to build the fill, except now it has a higher unit weight (because of compaction) and thus has been better engineering properties. This is a common

way

to

remediate

problems

with

collapsible

soils.

Removal also may be available option when the excavated soils have other problems, such as contamination or excessive organics, and need to hauled away. This method can be expensive because of the hauling costs and the need for imported soils to replace those that were excavated. It also can be difficult to find a suitable disposal site for the excavated soils.

Removal and replacement is generally practical only above the ground water table. Earthwork operations become more difficult when the soil is very wet, even when the free water pumped out, and

thus

are

generally

avoided

unless

absolutely

necessary.

2. PRECOMPRESSION OF SOIL

Another old and simple method of improving soils is to cover them with a temporary surcharge fill as shown in figure. This method is called precompression, preloading, or surcharging. It is especially useful in soft clayey and silty soils because the static weight of the fill causes them to consolidate, thus improving both settlement and strength properties. Once the desired properties have been obtained, the surcharge is removed and construction proceeds on improved site. Pre-compression



has

the

advantages

It requires only conventional equipment earthmoving equipment, which is readily available. No special or proprietary equipment is needed.



following

Any grading contractor can perform the work.



The results can be effectively monitored by using appropriate instrumentation and ground level surveys.



The method has a long track record of success.



The cost is comparatively low, so long as soil for preloading is readily available. However, there also are disadvantages



The surcharge fill generally must extend horizontally at least 10m beyond the perimeter of the planned construction. This may not possible for confined sites.



The transport of large quantities of soil onto the sites may not be practical, or may have unacceptable environmental impacts (i.e., dust, noise, traffic) on the adjacent areas.



The surcharge must remain in place for months or years, thus delay in construction.

3 DENSIFICATION TECHNIQUES

The strength and stiffness of the soil is higher when the particles are packed in a dense configuration than they are packed loosely. As a result, densification is one of the most effective and commonly

used means of improving soil characteristics. This can be approaches in following ways.

3.1 VIBRO TECHNIQUES

Vibro techniques use probes that are vibrated through soil deposit in a grid pattern to densify the soil over the entire area of thickness of the deposit. These are classified in to the following methods.

These

are

3.1.1VIBRO COMPACTION

Vibro compaction is a method for compacting deep granular soils by repeatedly inserting a vibratory

probe.

It

is

also

known

as

VIBRO

DENSIFICATION.

By inserting depth vibrators, the vibrations are produced by rotating a heavy eccentric weight with the help of an electrical motor with in the vibrator. The vibratory energy is used to rearrange the granular particles in a denser state. Penetration of the vibro probe is typically aided by water jetting at the tip of

the

The Vibro-Compaction Process

probe.

Some of advantages and disadvantages of this method are given below.

It is often an economical alternative to deep foundations, especially when considering the



added liquefaction protection in seismic areas.

o

It is most effective in granular soils

o

It cannot be used in cohesive soils

3.1.2. VIBRO FLOTATION

In vibro flotation a torpedo like probe (the vibro float) suspended by a crane is used to density a soil deposit.Vibro floats usually 12 to 18 inch in diameter and about10 to 16 ft long, contain weights mounted eccentrically on a central shaft driven by electric or hydraulic power. The vibro float is initially lowered to the bottom of the deposit by a combination of vibration and water or air jetting through ports in its pointed nose cone. The vibro float is then incrementally with drawn in 2 to 3 ft intervals at an over all rate of about 1ft / min while still vibrating. Water may be jetted through ports in the upper part of the vibro float to loosen the soil above the vibro float temporarily and aid in its with drawl. The vibrations produce a localized zone of temporary liquefaction

that

causes

Principle of the technique

the

soil

surrounding

the

vibro

float

to

densify.



Vibro flotation is most effective in clear granular soils with the fine contents less than 20% and clay contents below 3%.



Vibro flotation has been used successfully to density soils to deep [this of up to 115 ft.]

3.2. DYNAMIC COMPACTION:

Dynamic compaction is a ground i mprovement process for compacting and strengthening loose or soft soils to support buildings, roadways, and other heavy construction. The method involves the systematic dropping of heavy weights, 100 to 400kN, from a height of 5 to 30m, in a pattern designed to remedy poor soil conditions at the proposed building site. In soft ground areas, dynamic compaction has proved to be an effective and economical alternative to preloading, foundation piling, deep

vibratory

compaction,

and

soil

undercutting

Dynamic Compaction is normally used under the following circumstances:

and

replacement



To increase in-situ density and in this way improve the bearing capacity and consolidation characteristics of soils (or waste materials) to allow conventional foundation and surface bed construction to be carried out. The technique typically improves the in-situ soils such that allowable bearing pressures of up to 250 Kpa can be used with foundation settlements of the order



of

10

to

20

mm.

To increase in-situ density and in this way improve in-situ permeability and/or reduce liquefaction potential What soils are suitable?

Most soil types can be improved, including silts and some clays. The most commonly treated soils are old fills and granular virgin soils. Soils below the water table are routinely treated. However, careful control has to be used to allow dissipation of excess pore pressures created during the weight dropping. 3. 3. BLASTING

Blasting densification involves the detonation of multiple explosive charges vertically spaced at 10 to 20 ft apart in drilled or jetted bore holes. The bore holes are usually spaced between 15 to 50 ft apart and back filled prior to detonation. The efficiency of densification process can be increased by detonating the charges at different elevations at small time delays. Immediately after detonation, the ground surface rises & gas & water are expelled from fractures. The ground surface then settles as the excess gas & water pressure dissipates. Two or three rounds of blasting are

often

used

to

achieve

the

desired

degree

of

densification.



Blasting is most effective in loose sands that contain less than 20% silt and less than 5% clay.



Although blasting is quite economical, it is limited by several considerations, as it produces strong vibrations that may damage near by structures or produce significant ground movements.

3.4. COMPACTION GROUTOING

Compaction grouting uses displacement to improve ground conditions. A very viscous (low mobility) aggregate is pumped in stages, forming grout bulbs, which displace and densify the surrounding

soils.

A consistency soil cement paste is injected under pressure in to the soil mass, consolidating, and there by densifying surrounding soils in place. The injected ground mass occupies void space created by pressure-densification. Pump pressure transmitted through low mobility grout, produces compaction by displacing soil at depth until resisted by the weight of over lying soils.



Fine grained soils with sufficient permeability to allow excess water to dissipate best suits for compaction grouting.



It has also been used successfully in a wide variety of soils and fills.

4. REINFORCEMENT TECHNIQUES

In some cases it is possible to improve the strength and stiffness of a existing soil deposit by installing discrete inclusions that reinforce the soil. These inclusions may consist of structural materials, such as steel, concrete or timber and geomaterials such as densified gravel.

4.1. STONE COLUMNS

Soils deposits can be improved by the installation of dense columns of gravel known as stone columns. They may be used in both fine and coarse grained soils. In fine-grained soils, stone columns are used to increase the shear strength beneath structures and embankments by accelerating consolidation (by allowing radial drainage) and introducing columns of stronger

material. Stone columns can be installed in a variety of ways. (They may be constructed by introducing gravel during the process of vibroflotation) In the Frankie method, a steel casing initially closed at the bottom by a gravel plug is driven to the desired depth by an internal hammer. At that depth part of the plug is driven beyond the bottom of the casing to form a bulb of gravel. Additional gravel is then added and compacted as the casing is with drawn. The diameter of the resulting stone column depends on the stiffness and compressibility of the surrounded soil

4.2. COMPACTION PILES

Granular soils can be improved by the installation of compaction piles. Compaction piles are displacement piles , usually prestressed concrete or timber, that are driven into a loose sand or gravel

deposit

in

a

grid

pattern

and

left

there.

Compaction piles improve the seismic performance of a soil by three different mechanisms. First the flexural strength of piles themselves provides resistance to soil movement (reinforcement). Second, the vibrations and displacements produced by their installation cause densification. Finally, the installation process increases the lateral stress in the soil surrounding the piles. Compaction piles generally densify the soil with in a distance of 7 to 12 pile diameters and consequently are usually installed in a grid pattern. Between compaction piles a relative density of up to 75% to 80% are usually achieved. Improvement can be obtained with reasonable economy

to

depth

of

about

60ft.

4.3 DRILLED INCLUSIONS

Structural reinforcing elements can also be installed in the ground by drilling or auguring. Drilled shafts, some times with very large diameters, have been used to stabilize many slopes.

Soil nails, tie backs, micro piles have been used for this purpose. The installation of such drilled inclusions can be quite difficult. However in the loose granular soils that contribute to increase the

strength

of

the

soil

in

a

every

effective

manner.

5 GROUTING AND MIXING TECHNIQUES

Grouting techniques involve of cementitious materials into voids of the soil or into fractures in the soil so that the particle structure of the majority of the soil remains intact. Mixing techniques introduce cementitious materials by physically mixing them with the soil, completely disturbing the particle structure of the soil. Grouting and mixing techniques tend to be expensive but can often be accomplished with minimal settlement or vibration. 5.1

PERMEATION

GROUTING

Permeation grouting involves the injection of low viscosity liquid grout into the voids of the soil without disturbing the soil structure. Particulate grouts (i.e., aqueous suspensions of cement, fly ash, bentonite, micro fine cement or some combination there of) or chemical grouts (e.g., silica &

lignin

gels,

or

phenolic

&

acrylic

resins)

may

be

used.

Grout pipes are typically installed in a grid pattern at spacing of 4 to 8 feet. The grout may be injected in different ways. In „stage grouting‟, a boring is advanced a short distance before grout is injected through the end of the drill rod. After the grout sets up, the boring is advanced another short distance and grouted again. This process continues until grout has been placed to the desired

depth.

Permeation grouting produces soil improvement by two mechanisms. First the grout tends to strengthen the contacts between individual soil grains, there by producing a soil skeleton that is stronger and stiffer than that of the un grouted soil. Second, the grout takes up space in the voids between

soil

particles,

reducing

the

tendency

for

densification.



Stopping leaks in below-grade structures



Stopping leaks in below-grade utilities



Excavating support of non-corrosive soils



Strengthening of soil mass to accept new loads

5.2. JET GROUTING:

In Jet grouting the soil is mixed with cement grout injected horizontally under high pressure in a previously

drilled

bore

hole.

Jet grouting uses a special pipe equipped with horizontal jets that inject grout into the soil at high pressure. The pipes are first inserted to the desired depth, then they are raised and rotated while the



injection

is

in

progress,

thus

forming

a

column

of

treated

soil.

Because of high pressure, this method is usable on a wide range of soil types.

3.6. STABILIZATON USING ADMIXTURES

SOIL STABILIZATION: It is the process of improving the engineering properties of soil by

mixing some binding agents thus binding the soil particles .In a broader sense it also includes compaction, pre consolidation and many more such process. Soil stabilization is classified as



Mechanical stabilization



Chemical stabilization

3.6.1MECHANICAL

STABILIZATION

Mechanical stabilization is the process of improving the properties of soil by changing its gradation. Two (or) more types of natural soils mixed to obtain composite which is suspension to any

of

its

components

3.6.2. CHEMICAL STABILIZATION

Chemical stabilization is the form of lime, cement, fly ash and the combination of the above is widely used in soil stabilization to



Reduce the permeability of the soil.



Improve shear strength.



Increase bearing strength.



Decrease settlement. Soil and chemicals are mixed either mechanically in place or by bath process .the optimum benefit of using these agents in stabilization must be determined by laboratory testing. The

general principle of these admixtures as stabilizers is discussed below.

3.6.2.1.

LIME

STABILIZATION

This is done by adding lime to soil. It is useful for stabilization of clayed soils. When lime reacts with soil, there is exchange of cations in the adsorbed water layer and a decrease in plasticity of  soil occurs .The resulting material is more friable than the original clay and is therefore more suitable

as

sub

grade.

This method is not effective for sandy soils. However these soils can be stabilized in combination with clay, fly ash or other pozzolanic materials, which serve hydraulically reactive in

gradients.

3.6.2.2.CEMENT STABILIZATION Cement stabilization is done by mixing pulverized soil

and Portland cement with water and compacting the mix to attain a strong material .The material obtain by mixing soil and cement is known as soil cement .The mix becomes hard and durable structural

material

as

the

cement

hydrates

and

develops

strength.

The soil cement is quite weather resistant and strong. It is commonly used for stabilizing sandy and other low plasticity soils. Cement interacts with the silt and clay fractions and reduced their affinity

for

3.6.2.3.FLY

water

.It

reduces

the

ASH

swelling

characteristics

of

the

soil

.

STABILIZATION

Fly ash is a by product of the pulverized coal combustion process. Fly ash has silica, alumina and various oxides and alkalis as its constituents .It is fine grained and pozzolanic in nature. Fly ash reacts actively with hydrated lime and hence is used in combination with lime as a stabilizer. A mixture of about 10 to 35 % of fly ash and 2 to 10 % of lime forms as effective stabilizer for stabilization of highway bases and sub bases .Soil-lime-fly ash mixes are compacted under controlled

condition

with

adequate

quantity

of

3.7.

water.

GEOTEXTILES

Soil conditions can be improved in an excellent manner by using geo textiles. Geotextiles are porous fabrics manufactured products and others such as polyester ,polyethylene, polypropylene and polyvinylchloride, nylon, fiber glass and various mixtures of these. These are having permeabilities

comparable

in

range

from

coarse

gravel

to

fine

sand.

Geotextiles have been used in a variety of civil engineering works. Thus in the selection of a proper geotextile, due importance has to be given to the major function that the geotextile is intended

to

perform.

These

are

majorly

used

as

follows.

1. They acts as separators between two layers of soils having a large difference in particle size to prevent

migration

of

small

size

particles

into

the

voids

of

large

size

particles

2. They act as filter. When the silt laden turbid water passes through the geotextile, the silt particles

are

prevented

from

movement

by

the

geotextile.

3. Geotextiles themselves function as a drain because they have a high water transporting capacity

than

that

of

the

surrounding

material.

4. They serve as REINFOREMENT in soil since they are a good in tensile strength.

3.7. ELECTRO OSMASIS AND ELCTRO CHEMICAL HARDENING METHOD

The electroosmasis process can be used to increase the shear strength and reduce the compressibility of soft clayey and silty soils beneath foundation. By introducing an electrolyte such as calcium chloride at the anode, the base exchange reaction between the iron anode and surrounding soil is increased, resulting in the formation of ferric hydroxides which bind the soil particles together. However because cost of electric power and wastage of electrodes, electroosmasis with or without electrochemical hardening can be considered only for special situations

where

the

alternative

of

piling

cannot

be

adopted.

4. CONCLUSION

1.

Unfavorable soil conditions can frequently be improved using soil improvement techniques.

A variety of soil improvement techniques have been developed. However a suitable technique has 2.

to

be

adopt

according

to

necessity

of

the

structure

and

economy.

Mainly soil improvement techniques can be divided in to four broad categories;

Densification technique, Reinforcement technique, grouting or mixing technique and stabilization technique. 3.

Densification is probably the most commonly used soil improvement technique. Most

densification techniques relay on tendency of granular soils to densify when subjected to vibrations. However there is a possibility of damaging adjacent structures and pipelines due to application of this technique.

1.

Reinforcement techniques introduce discrete inclusions that stiffen and strengthen a soil deposit. The high stiffness and strength of the inclusions also tend to reduce the stresses imposed on the weaker material between the inclusions.

2.

Grouting techniques involve the injection of cementitious materials into the voids of the soil or into fractures of the soil, so that the particle structure of the majority of soil remains inject. In permeation grouting, very low viscosity grouts are injected intothe voids of the soil with out disturbing the soil structure. In intrusion grouting, thicker and more viscous grouts are injected under pressure to cause controlled fracturing of the soil.

3.

Now a days, geotextiles are extensively used for improving the soil conditions. These have multiple applications as they act as filters, reinforcement, separations…etc.

5. BIBILOGRAPHY

1.

”Geotechnical Engineering Principles & Practices” by Donald P .Coduto

1.

“Foundation Design & Cinstruction “by M.J.Tomlinson.

2.

“Geotechnical Engineering” by Purshotham raj

3.

“Geotechnical Earthquake Engineering” by Steven L.Kramar.